The envelopes or arc tubes of high pressure sodium vapor lamps are generally made of alumina ceramic which can withstand the attack of sodium at high temperatures. The ends of the ceramic tubes may be sealed by metal end caps which serve as conductors and terminals for the electrodes, or by ceramic end plugs through which metal conductors are sealed. The refractory metals used are niobium and tantalum whose coefficients of thermal expansion are reasonably close to those of alumina ceramic from room temperature up to about 700 to 800.degree. C which is the operating temperature of the seals in a high pressure sodium vapor lamp.
A sealing frit is used to cement the metal parts to the ceramic but it does not adhere well when applied directly to niobium or tantalum. However, when niobium is first coated with a sintered layer of tungsten and tungsten oxides and the sealing frit applied thereover as disclosed in U.S. Pat. No. 3,448,319 - Louden, a good seal is obtained. The tungsten and tungsten oxide coating tends to increase the hardness of the niobium and make it brittle, but this can be remedied by coating the outside of the end cap with zirconium hydride as taught in U.S. Pat. No. 3,479,170 - Louden. While the Louden seal is reliable and has been used commercially up to the present, its high labor content makes it relatively expensive and a cheaper seal is desired.
In another seal disclosed in U.S. Pat. No. 3,598,435 - Jorgensen, the refractory metal surface to be bonded to the alumina body has a layer of zirconium applied to it which forms an oxide upon contact that is incorporated into the sealing glass whereby adherence is promoted. The zirconium layer may be formed by coating the niobium with zirconium hydride which is then reduced to zirconium, or alternatively by zirconiding wherein an adherent coating of zirconium is formed on a niobium surface by electrodeposition in a fused salt bath. Notwithstanding the excellence of the Jorgensen seal, it has not been used commercially because the available processes, including the two previously mentioned, for forming a zirconium coating on a refractory metal such as niobium or tantalum were too involved and expensive.